Impact testing is often conducted for aircraft components to test for the amount of damage caused by events, such as bird strikes, that are expected to occur during aircraft operation. For bird-strike testing of, for example, aircraft transparencies (such as the windscreen of a helicopter canopy), fuselages, or blades, an air cannon may be used to launch a bird carcass or a gelatin substitute at a selected velocity.
When firing a semi-solid projectile from an air cannon during high-speed impact testing, the projectile, although it has a diameter similar to the ID of the barrel, does not create a solid seal because of its semi-solid characteristics. This leads to the use of sabots to drive the projectile forward within the barrel, thereby forming a more reliable and repeatable seal and minimizing the deformation of the projectile as it is accelerated during firing. Such a system is shown and described in ASTM Standard F330-16, titled “Standard Test Method for Bird Impact Testing of Aerospace Transparent Enclosures.”
FIGS. 1 and 2 illustrate components of a prior-art air cannon. Air cannon 11 comprises an air tank 13, which is connected to a pressurized air source by line 15, and a cylindrical barrel 17 having a bore 19. A cylindrical sabot 21 with a forward pocket 22 is configured for being propelled forward through barrel 17 and away from tank 13 by the air pressure in tank 13, sabot 21 having an outer diameter smaller than the diameter of bore 19. After sabot 21 exits barrel 17, a sabot stripper 23 attached to the forward end of barrel 17 stops the forward motion of sabot 21 and retains sabot 21 within stripper 23.
FIG. 2 is a cross-section view of stripper 23, which is attached to barrel 17 adjacent muzzle 25. Stripper 23 comprises a body 27 formed from perpendicular plates 28. A conical cavity 29 within body 27 is truncated at the forward end by a bore 31. The rear diameter of cavity 29 is larger than bore 19 of barrel 17, allowing sabot 21 to pass into cavity 29, but the diameter of bore 31 is smaller than the outer diameter of sabot 21. The forward end of sabot 21 strikes the inside of cavity 29 at approximately the line indicated by numeral 33, and sabot 21 is retained within cavity 29 as projectile continues forward and exits stripper 23 through bore 31.
Current sabots are fired together with the projectile and either separate during flight or are caught at the end of the barrel with the sabot stripper. Both types of sabots are typically too damaged to be reused or not capable of multiple shots without major repairs. Also, cannons using sabot strippers require the sabot to be significantly larger than the projectile to ensure that the sabot is caught by the sabot stripper. This means that the barrel is larger than needed to house the projectile, adding unnecessary weight and size to the system. Also, the barrel may require reinforcement to accommodate the forces generated when catching a sabot at full velocity (>140 kn).
Alternative versions of sabots deform or break apart in flight, such as expanding cup, base, spindle, and ring sabots. However, in order to maintain the level of accuracy needed for impact testing in aviation (typically required to be within 1″ of the target), projectiles are fired from a minimum required distance, and this is often approximately 5 feet from the target. This distance between the end of the barrel and the target is too short for a sabot to reliably come apart and clear the target during flight.